1. Field of the Invention
This invention relates to a plasma CVD (chemical vapor deposition) process and a plasma CVD system which make use of high-frequency power and are usable in the manufacture of semiconductor devices, electrophotographic photosensitive member devices, image-inputting line sensors, flat-panel display devices, image pickup devices, photovoltaic devices and so forth.
2. Related Background Art
In recent years, in the process of producing semiconductor devices and the like, plasma CVD systems and plasma CVD processes have been put into practical use in an industrial scale. In particular, plasma CVD systems making use of a high-frequency power of 13.56 MHz are in wide use because processing can be carried out regardless of whether substrate materials and deposited-film materials are conductors or insulators.
As an example of conventional plasma-producing high-frequency electrodes and plasma CVD systems and processes making use of such electrodes, a parallel-plate type system will be described with reference to FIG. 1. In a reactor 101, a high-frequency electrode 103 is provided via an insulating high-frequency electrode support base 102.
The high-frequency electrode 103 is a flat plate provided in parallel to an opposing electrode 105, and plasma is caused to take place by the aid of an electric field determined by electrostatic capacitance exhibited between the electrodes. Once plasma has taken place, a plasma region which is substantially a conductor and a sheath which acts chiefly as a capacitor in an equivalent manner between the plasma and the both electrodes or reactor wall are formed between the electrodes to provide an impedance greatly different from that before the plasma takes place.
Around the high-frequency electrode 103, an earth shield 104 is provided so that any discharge may not occur between the side of the high-frequency electrode 103 and the wall of the reactor 101. To the high-frequency electrode 103, a high-frequency power source 111 is connected through a high-frequency power supply wire 110.
A flat-plate film-forming substrate 106 on which plasma CVD is carried out is attached to the opposing electrode 105 provided in parallel to the high-frequency electrode 103, and the substrate 106 to be processed is kept at a desired temperature by a substrate temperature control means (not shown).
Plasma CVD using this system is carried out in the following way. After the inside of the reactor 101 is evacuated to a high vacuum by an evacuation means 107, reaction gases are fed into the reactor 101 through a gas feed means 108, and its inside is kept at a predetermined pressure. A high-frequency power is supplied from the high-frequency power source 111 to the high-frequency electrode 103 to cause a plasma to take place across the high-frequency electrode and the opposing electrode.
Thus, the reaction gases are decomposed and excited by plasma to form a deposited film on the film-forming substrate 106. As the high-frequency power, it is common to use a high-frequency power of 13.56 MHz. Use of such a discharge frequency of 13.56 MHz makes it relatively easy to control discharge conditions and brings about an advantage that the film formed can have a good film quality, but may result in a low gas utilization efficiency and a relatively small deposited-film formation rate.
Taking account of these points, studies are made on plasma CVD carried out at a high-frequency power having a frequency of about 25 to 150 MHz. For example, Plasma Chemistry and Plasma Processing, Vol. 7, No. 3, 1987, pp.267-273 (hereinafter "publication 1") discloses that a material gas (silane gas) is decomposed by a high-frequency power having a frequency of about 25 to 150 MHz, using a parallel-plate type glow discharge decomposition system.
Stated specifically, the publication 1 discloses that, in the formation of a-Si films at frequencies changed within the range of from 25 MHz to 150 MHz, film deposition rate reaches a maximum of 2.1 nm/sec when 70 MHz is used, and this is a formation rate about 5 to 8 times that in the plasma CVD carried out at 13.56 MHz, and that a-Si film defect density, optical band gap and conductivity are not so much affected by excitation frequencies.
The publication 1 shows an example of a plasma CVD system suited for the processing of flat substrates of a laboratory scale. As for an example of a plasma CVD system suited for the formation of deposited films on film-forming substrates of a large industrial scale (e.g., cylindrical substrates), it is disclosed in, e.g., U.S. Pat. No. 5,540,781 (hereinafter "publication 2").
This publication 2 discloses a plasma CVD process and a plasma CVD system which make use of a high-frequency power of what is called VHF band, having a frequency of from 60 MHz to 300 MHz. The plasma CVD system as disclosed in the publication 2 will be described with reference to FIG. 2.
The plasma CVD system shown in FIG. 2 is the VHF plasma CVD system disclosed in the publication 2.
In FIG. 2, reference numeral 200 denotes a reactor. The reactor 200 has a base plate 201, insulating members 202A, cathode electrodes 203C, insulating members 221B, cathode electrodes 203B, insulating members 221A, cathode electrodes 203A, insulating members 202B and a top cover 215.
Reference numeral 205A denotes a substrate holder, which has a heater column 205A' inside. Reference numeral 205A" denotes a substrate heater attached to the heater column 205A'. Reference numeral 206 denotes a cylindrical film-forming substrate provided on the substrate holder 205A. Reference numeral 205B denotes an auxiliary holding member for the cylindrical film-forming substrate 206. The substrate holder 205A has at its bottom a rotating mechanism (not shown) connected to a motor and is so designed as to be optionally rotatable. Reference numeral 207 denotes an exhaust pipe having an exhaust valve, and the exhaust pipe communicates with an exhaust mechanism 207' having a vacuum pump. Reference numeral 208 denotes a material gas feed assemblage constituted of gas cylinders, mass-flow controllers, valves and so forth. The material gas feed assemblage 208 is connected to gas release pipes 216 having a plurality of gas release holes, through a gas feed pipe 217. Material gases are fed into the reactor through the plurality of gas release holes of the gas release pipes 216. Reference numeral 211 denotes a high-frequency power source, and a high-frequency power generated here is supplied to the cathode electrodes 203 (203A to 203C) through a high-frequency power supply wire 218 and matching circuits 209 (209A to 209C). In the plasma CVD system shown in FIG. 2, the cathode electrodes are so constituted as to be divided electrically into three electrodes 203A, 203B and 203C in the axial direction of the cylindrical film-forming substrate. The high-frequency power generated in the high-frequency power source 211 is divided into three parts by a high-frequency power dividing means (distributor) 220, and then supplied to the cathode electrodes 203A, 203B and 203C through matching circuits 209A, 209B and 209C, respectively.
The publication 2 also describes a plasma CVD process carried out using the plasma CVD system shown in FIG. 2.
That is, in the system shown in FIG. 2, the cylindrical film-forming substrate 206 is set to the substrate holder 205, and thereafter the inside of the reactor 200 is evacuated by the operation of the exhaust mechanism 207' to evacuate the inside of the reactor to have a predetermined pressure. Then, the heater 205A" is electrified to heat the substrate 206 so as to be kept at a desired temperature.
Next, material gases are fed into the reactor 200 from the material gas feed assemblage 208 through the gas feed pipe 217 and gas release pipes 216, and the inside of the reactor is adjusted to a desired pressure. In this state, a high-frequency power having a frequency in the range of from 60 MHz to 300 MHz is generated by the high-frequency power source 211. The high-frequency power is divided into three parts in the high-frequency power distributor 220, and then supplied to the cathode electrodes 203A, 203B and 203C through the matching circuits 209A, 209B and 209C, respectively. Thus, in the space defined by the cylindrical film-forming substrate 206 and the cathode electrodes, the material gases are decomposed by high-frequency energy to produce active species, so that a deposited film is formed on the cylindrical film-forming substrate 206.
The publication 2 states that, since the cylindrical cathode electrode is divided in the plasma CVD system making use of the high-frequency power having a frequency in the range of from 60 MHz to 300 MHz as stated above, a highly uniform deposited film can be formed on a large-area cylindrical film-forming substrate while maintaining the high film deposition rate that is an advantage of the VHF region high-frequency plasma CVD.
However, the film formation using the high-frequency power having a frequency of from 25 to 150 MHz in the parallel-plate type system disclosed in the publication 1 is carrie out in a laboratory scale, and also the publication does not refer at all to whether or not such an effect can be expected in the formation of large-area films. In general, the higher the excitation frequency is, the more remarkable the influence of standing waves produced on a high-frequency electrode is, where, especially on flat electrodes, two-dimensional complicated standing waves may occur. Hence, it is foreseen that it will be difficult to form large-area films uniformly.
In the plasma CVD process and plasma CVD system disclosed in the prior art publication 2, it can be expected that deposited films are formed at a high deposition rate and in a high uniformity when large-area deposited films are formed in a cylindrical form. However, it is foreseen that a plurality of feeding points will be required on one cathode to make the system complicated and also that it will be difficult to make adaptation to flat substrates.